This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Swedish Application No. 0004813-2 filed on Dec. 21, 2000, the entire content of which is incorporated herein by reference.
The present invention relates to improved rolling bearings, in particular bearings being used in aqueous environment, such as water and steam. Such bearings typically comprising, for example, an outer race, an inner race, a number of rolling elements, and a retainer holding said rolling elements in a spaced relation from each other, and which the bearing resists undue wearing.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Existing bearing systems are today based on steel, stainless steel, hybrid bearings (silicon nitride rolling elements) and all-ceramic (races and rolling elements of silicon nitride). These bearings systems using steel and silicon nitride ceramics all fail prematurely if water is present. These observations have been gained both from laboratory work, as well as in customer applications.
The difficulty with water is based on the fact that the water molecule is very small, whereby it enters microscopic cracks and cause embrittlement of most ceramics, containing a glassy phase, and steels. Water also cause polishing wear by surface oxidation of non-oxide ceramics, and crevice corrosion at standstill. Also, water does not show any tendency of forming elastohydrodynamic lubrication (EHL) films, leading to wear by surface-to-surface contacts.
It has been recognized that in order to solve these problems, bearings built with all ceramics, or hybrid bearings, having rolling elements of ceramics may have races of another material, e.g., steel might be useful. However, it is still the reduced ability of water to build thin EHL films at the contact between retainer and rolling elements of a ceramic material that creates a problem and in particular at high rotational speeds, thus such solutions have proved insufficient.
It is previously known (U.S. Pat. No. 5,271,679) to produce a rolling element bearing comprising outer and inner races, ball elements, and a retainer to keep the ball elements spaced from each other, whereby the retainer consists of a mixture of polytetrafluoroethylene, MoS2, and WS2, aramid fibres, and a polyether ether ketone (PEEK) resin. This bearing is intended to be used in air or vacuum in an environment having high or low temperature, in the presence of radioactive rays, or in any other environment not permitting the use of any lubricating oil or grease. The retainer material provides the lubricating material to the bearing. Outer and inner raceways, as well as the balls present are made of stainless steel.
JP-A-90-87025 relates to a ball bearing wherein at least a part of the inner and outer runways is made of or coated with cermet, stellite, or precipitated stainless steel, and at least the surface parts of the balls are formed of one kind of the materials selected from cermet, stellite, and ceramics composed mainly of silicon carbide. The runways and the rolling elements are made of different materials and so selected that the rolling elements possess the hardest surface. The bearing is intended to be used in water at elevated temperatures and high pressure.
DE-A-4,235,838 relates to a bearing wherein at least one of the inner runway, outer runway and rolling elements are made of a ceramic material consisting of sintered silicon nitride comprising 0.5 to 40% by weight of Mg, 0.3 to 3% by weight of Zr, and 1.5 to 5.0% by weight of a material selected from Sr, Al and a rare earth metal, and the reminder is silicon nitride. The metals are present as oxides. The bearing is intended to be used in a corrosive environment such as a melting furnace, metal melt plating bath or electro plating bath while molding alloys.
DE-A-4,207,034 relates to a bearing having an outer runway made of stellite. The bearing is intended for use in a device used in a bath of melt metal.
EP-A-0 492 660 relates to a retainer of a bearing which retainer consists of 15 to 50% by weight of glass fibres or the similar reinforcing fibres, 5 to 15% by weight of carbon fibres, and a balance of polyether ether ketone (PEEK). The bearing is intended for use at high temperatures.
DE-A-19,606,249 relates to a bearing retainer of polyimid resin in which 5 to 10% by weight is polytetrafluoroethylene (PTFE) and 10 to 20% by weight is graphite. The bearing is intended for use in high speed tools (300,000 to 400,000 rpm). The bearing is to be lubricated using paraffin.
U.S. Pat. No. 4,906,110 relates to a roller bearing having solid lubricants, whereby the retainer is made of a polyimid resin containing a selected solid lubricant.
A problem to be solved is to provide a bearing system that will work in water and/or steam environment from ambient temperature up to 100-300xc2x0 C. and thereby withstand various types of water such as salt water, water contaminated or water based liquids. Initial bearing types are those used, e.g., in sea-water pumps, oil drilling heads, fire resistant hydraulic liquids, and compressors.
An object of the present invention is to obtain a rolling element bearing comprising a combination of materials suitable for aqueous environment, such as water, process fluids including water, oils with water contamination, fire resistant hydraulic liquids, steam, and the similar.
According to one aspect, the present invention provides a rolling bearing for use in an aqueous environment comprising: an outer race, an inner race, rolling elements, and a retainer holding said rolling elements in a spaced relation from each other, wherein the races of the bearing are made of stellite, the rolling elements are made of hot-isostatic-pressed alumina-zirconia composites, and the retainer is made of polyether ether ketone comprising graphite fibres, graphite powder, and polytetrafluoroethylene.